[0010] The invention relates to the field of radiation therapy treatment
planning.

[0011] BACKGROUND AND SUMMARY OF THE INVENTION

[0012] For most treatment modalities in radiation oncology, a 3D image of
the patient's internal anatomy is usually obtained using CT scan, MR
scan, ultrasound, PET scan or other imaging techniques. A physician,
physicist, or dosimetrist then contours (i.e. draws) outlines of patient
anatomy in a treatment planning system. All relevant anatomical
structures are contoured; including tumor targets as well as critical
organs. The treatment planning system is then used to determine the
expected radiation dose distribution throughout the 3D image
representation of the patient. From this overall 3D dose distribution,
the treatment planning system computes the dose to all the contoured
anatomical structures. In the prior art, the dose to all contoured
anatomical structures is summarized in the form of a Dose Volume
Histogram (DVH), which is a plot of volume versus dose, or alternatively,
a plot of dose versus volume. The embodiment disclosed in this
description is of the first form, but it also applies to the dose versus
volume form, just by exchanging the x and y axes.

[0013] One slice of a CT scan is shown in FIG. 1 with dose lines overlaid.
The entire CT scan consists of numerous such 2D slices, comprising the
entire 3D volume of the target area within the patient. The treatment
planning system determines how many points in each contoured anatomical
structure receive each particular dose value, and it organizes this
information into a DVH, as shown in FIG. 2.

[0014] As the prior art abundantly shows, dose tolerance limits and
computerized treatment planning systems that calculate DVHs have been in
existence for more than twenty five years. However, until the presently
disclosed invention there is still no treatment planning system that
overlays the dose tolerance limits onto the DVH and warns the user if any
limits have been exceeded. The conventional radiation dose tolerance
limits (Emami et al 1991) relate to a large volume of the contoured
anatomical structure, such as 1/3 of the total volume, 2/3 of the total
volume, or the dose to the entire volume. These large volumes can
visually be seen on the DVH, so although it would be more convenient and
comprehensive to use a system like the present invention, clinical
practitioners have become accustomed to doing this manually for the past
twenty five years.

[0015] The stereotactic body radiation therapy (SBRT) dose tolerance
limits (Wulf et al 2001, Chang & Timmerman 2007) are dramatically
different, however. Whereas in conventional radiation therapy treatments
the goal is to deliver a uniform dose over a fairly large target, the
goal of SBRT is to deliver a much higher dose per treatment to a small,
focal, precisely defined target (Lax 1993, Lax et al 1994, Murphy & Cox
1996) with much steeper dose gradients. The SBRT dose tolerance limits
are typically for a much smaller volume, like five cubic centimeters (5
cc), or 1 cc, or even the maximum point dose. These very small volumes of
dose are not as easily visualized on the DVH making the presently
disclosed invention much more important for patient safety--yet still
this has been performed manually in all clinics that have treated with
SBRT for the past fifteen years, probably due to clinical habits from
conventional radiation therapy.

[0016] An example DVH of volume versus dose is shown in FIG. 2 for an SBRT
treatment near a patient's small bowel. The disclosed invention works
equally well for any contoured anatomical targets or critical structures;
small bowel is just presented as an example to facilitate explanation.
The disclosed invention overlays dose limits onto the DVH, as shown in
FIG. 3 for the example in FIG. 2. The quality of the treatment plan in
FIG. 2 is very subjective because there is no concise information to
indicate whether the plan is good or not. However, it is much more useful
to display the plan as in FIG. 3, where it becomes clear that a certain
volume of small bowel exceeds a preset dose limit

[0017] In FIGS. 4 and 5 another example is shown, in which case multiple
limits are overlaid on a single DVH plot. In FIG. 5 it may be clearly
seen that the example esophagus dose from the SBRT treatment is below two
of the limits but above the third limit, whereas the prior art FIG. 4 is
more hazardous to patients because no dose limits are explicitly shown.
Cursory manual inspection of the DVH in FIG. 4 looks fairly good, but the
present invention clearly shows in FIG. 5 that the esophagus dose exceeds
the maximum point dose limit by 10.8 Gray. Although only a very small
volume, 0.24 cc of esophagus exceeds the limit, it is such a high dose
that the patient could be at increased risk of esophageal fistula. This
invention has the potential to improve patient safety by clearly warning
clinical practitioners whenever the dose to contoured anatomical
structures is too high, thereby showing which parts of the treatment plan
need to be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is one slice of a treatment plan for a patient with dose
lines overlaid on the CT scan.

[0025] A DVH can be expressed as a plot of {right arrow over (x)}, {right
arrow over (y)}, where {right arrow over (x)} is a vector of the range of
doses in the plan, from the minimum dose to the maximum dose, and {right
arrow over (y)} is the corresponding vector of the volumes of the
anatomical structure receiving each particular dose. The dose {right
arrow over (x)} and the volume {right arrow over (y)} may be expressed in
any applicable units, either absolute units or in normalized relative
units.

[0026] Dose tolerance limits may be expressed in three different formats:

[0027] A) Only volume YA of a specified structure may exceed dose
XA.

[0028] B) Only YB percent of a specified structure may exceed dose
XB.

[0030] Limit format C specifies that the maximum dose of the specified
structure may not exceed dose Xc. All three limits may be expressed
in the format:

[0031] Only Yi of a specified structure may exceed dose Xi,
where i is chosen from the set {A, B, C}, and the units of Yi are
volume or percent, and the units of Xi is dose. Using this notation,
the disclosed invention may be described by the flowchart in FIG. 7.

[0032] The points Xi, Yi could be displayed as a cross, plus,
dot, or asterisk, or plotted in any manner that clearly visible and
understandable to the user. We have found it convenient to plot the
points as an "L" shape, where the corner of the "L" is plotted at the
point Xi, Yi, and the tails upward and to the right emphasize
the relative level of the dose tolerance limit to the DVH, as seen in
FIG. 3 and FIG. 5. We stress, however, that this is simply one of many
embodiments; the invention includes all manner of displaying the points
Xi, Yi.

[0033] A flowchart of the disclosed invention is in FIG. 7. The user can
export the DVH data 720 from the treatment planning system into a
computer file, or the treatment planning system could call the presently
disclosed invention as a function call. In either case, the first import
module 710 imports the DVH data 720. The user can store the dose
tolerance limits 740 in a computer file, or another program could enable
the user to specify the dose tolerance limits 740 and call the disclosed
invention as a function call. In either case, the second import module
730 loads the dose tolerance limits 740. Module 750 plots the DVH data
720. Module 760 then overlays the dose tolerance limits 740 onto the same
plot. Finally, if any dose tolerance limits 740 have been exceeded,
module 770 warns the user.

[0034] The warning to the user could be provided in many ways: a textual
message could be displayed, or the DVH or dose tolerance limit could be
changed to a certain color, or an asterisk or other marker could be
displayed, the background color could change, and audible sound could be
used, or various other means could be employed to warn the user.

[0035] An example is shown in FIG. 5. The plot of the DVH data 720 is
curve 510. In this example, three dose tolerance limits 740 have been
specified by the user, and they are curves 520, 530, and 540. The DVH is
below two of the three dose tolerance limits since curves 520 and 530 do
not cross the DVH curve 510. However, the third dose tolerance limit,
represented by curve 540, does cross the DVH curve 510, so this dose
tolerance limit has been exceeded by the DVH data 720. Therefore, the
warning message 550 has been displayed to warn the user of the excessive
dose.

[0036] While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment,
it is to be understood that the invention is not to be Limited to the
disclosed embodiment, but on the contrary, is intended to cover various
modifications and equivalent arrangements included within the spirit and
scope of the appended claims.